Infra-red imaging systems and other optical systems

ABSTRACT

An infra-red imaging system has two variable focal length optical components ( 6, 7 ) whose positions are fixed relative to one another. The system also comprises an image detector ( 8 ). Control means ( 13 ) is provided such that the focal length of one of the variable focal length optical components ( 7 ) can be varied in relation to the focal length of the other variable focal length optical component ( 6 ). The system can be used to produce an image of variable magnification while maintaining an in-focus image at the detector ( 8 ). The control means ( 13 ) may comprise a mechanical linkage, an electronic circuit, or a computer program. In an alternative embodiment the optical system is a beam expander. Controlling a focusing mirror can achieve a dither effect. Controlling a focusing mirror can also de-focus the image, giving a mean scene temperature evaluation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to infra-red (e.g. thermal) imaging systems andto other optical systems, and in particular to optical systems in whichat least two optical parameters can be controlled relative to oneanother.

2. Discussion of Prior Art

It is known to produce optical systems comprising two or more opticalcomponents of fixed focal length in which variation of at least twooptical parameters of the system is achieved by moving two or more ofthe optical components relative to one another.

For example it is known to produce a variable magnification compoundlens comprising at least two optical components of fixed focal lengthwherein the overall optical magnification of the system is varied byadjusting the spacing between the optical components whilst an in-focusimage is maintained on a fixed image plane by relatively varying thedistance from the final optical component to the fixed image plane.

The technology used in such systems is well developed and providesacceptable results in most applications. However, the mass and responsetime of these systems can be adversely affected by the need tophysically move the optical components. Furthermore, for criticalapplications, especially in spacecraft optics, moveable opticalcomponents require complex counterbalancing arrangements.

It has also been known for nearly ten years from U.S. Pat. No. 4,836,661to propose a refractive variable magnification system for zoom lenses inwhich a number of refractive lenses of variable refractive power areprovided a fixed distance apart and are used to focus an image onto afixed image plane.

U.S. Pat. No. 4,890,903, published in 1990, discloses a lens unit havingvariable focus refractive lenses which can be bodily rotated to altertheir focal length, and which are gas or fluid filled. One lens have apositive power and the other a negative power. Uses in spectacles andcameras are disclosed.

U.S. Pat. No. 4,630,903 shows a complex multi-refractive lens system fora photocopies. it requires the ability to vary 3 system parameters from6 in a refractive system.

It has also been known for many years in the field of thermal imaging tohave detector, usually pixellated, upon which an image is focused andprovided with a chopper whose function is to cause that image to ditherover the detector, and to present a periodic reference signal to thedetector. A typical chopper has three segments: a first transmissiveprism, a second transmissive prism, and an opaque shutter region, thetwo prism sections causing an image to fall upon slightly differentregions of the detector, and the shutter region providing a black bodyreference signal which can be used to allow the output voltage of apixel to fall away from a scene-dependent value.

A paper by N. Butler, J. McClelland and S. Iwasa, employees ofHoneywell, entitled “Ambient Temperature Solid State Pyroelectric I.R.Imaging Arrays”, dated March 1988, NE 8701-02 (SPIE), discusses using ade-focusing chopper having a thick transparent “blurring” portion.

It has also been known to compensate for movement of a detected imagerelative to a sensor/sensor array by introducing special, andadditional, components into the optics of an imaging system, for exampleto compensate for atmospheric alteration, and camera shake.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan optical system comprising two or more variable focal length opticalcomponents whose positions are fixed relative to one another; the systemfurther comprising control means for varying the focal length of atleast one of the said variable focal length optical components inrelation to the focal length of at least one of the other said variablefocal length optical components such that, in use, control over at leasttwo optical parameters if the system is achieved.

The invention overcomes the drawbacks of much of the prior art becausevariation of certain optical parameters of the system can be achievedwithout the need to change the relative positions of the opticalcomponents within the system, i.e. without the need to, for example,move the optical components bodily, as a whole, towards or away fromeach other, or to rotate bodily the optical components. As a resultsystem reprogramming can be achieved in time-scales shorter than thoserequired for physical movement of the positions of the opticalcomponents, this being of particular benefit in active optical systems.(By active optical system it is meant an optical system in which thefocal lengths of the optical components are varied in real timeaccording to predictions regarding the required focal lengths, asopposed to being controlled by a feedback loop.) Furthermore, if thepower supplies required for varying the focal length of the opticalcomponents are of low-mass design, then the overall mass of the systemmay be lower than for a conventional system due to the lack ofmechanical positioning gear. Also the need for complex counterbalancingrequirements is eliminated for critical applications such as spacecraftoptics.

The optics may be used in a beam expander; or in a zoom lens unit; or ina camera; or in binoculars or a telescope. The optics may be reflective.An all-reflective optics device may be provided.

The control means in an optical system according the first aspect of theinvention may comprise a mechanical linkage, electronic circuit or acomputer program.

An optical system according to the first aspect of the invention mayfurther have an image plane fixed relative to the said variable focallength optical components in the system.

One optical parameter that may be controlled is the magnification of thesystem. Alternatively, the focal lengths of the first and secondcomponents may be the two controlled parameters. Tilt means may beprovided to apply a two-axis tilt to one or more of the opticalcomponents. De-focusing means may be provided to de-focus the imagereceived by the detector/detector array.

In one application of an optical system according to the first aspect ofthe invention, variation of the focal length of at least one of the saidvariable focal length optical components in relation to the focal lengthof at least one of the other said variable focal length opticalcomponents varies the position of a principle plane of the system independence on a change in effective focal length of the compound systemsuch that an image of variable magnification is obtained withmaintenance of a substantially in-focus image in a fixed image plane.

In a further application of an optical system according to the firstaspect of the invention, variation of the focal length of at least oneof the said variable focal length optical components in relation to thefocal length of at least one of the other said variable focal lengthoptical components may vary the width of an optical beam whilstsubstantially maintaining collimation of the beam.

According to a second aspect of the present invention there is providedan optical system for varying the width of an optical beam, comprisingtwo or more variable focal length optical components whose positions arefixed relative to one another, and control means for varying the focallength of at least one of the said variable focal length opticalcomponents in relation to the focal length of at least one of the othersaid variable focal length optical components such that, in use, thewidth of the optical beam is varied whilst substantially maintaining thecollimation of the beam.

The width of the optical beam may be expanded, or may be contracted.

The optical components may comprise refractive optical components, ormay comprise reflective optical components, such as mirrors.

According to a third aspect of the invention there is provided avariable magnification zoom lens unit comprising two or more variablefocal length optical components whose positions are fixed relative toone an other and to an image plane of the lens unit; the lens unitfurther comprising means for varying the focal length of at least one ofthe variable focal length optical components relative to the focallength of at least one other of the variable focal length opticalcomponents such that, in use, an image of variable magnification can beobtained with maintenance of a substantially in-focus image in the fixedimage plane.

According to a fourth aspect of the invention there is provided a methodof controlling at least two optical parameters of an optical systemhaving two or more optical components comprising the steps of:

a) fixing the positions of at least two of the optical componentsrelative to one another; and,

b) relatively varying the focal length of at least two of the said fixedposition optical components.

According to a fifth aspect of the present invention there is providedan optical system comprising at least first and second variable focallength reflective optical components whose positions are fixed relativeto one another; the system further comprising control means for varyingthe focal length of the first optical component and for varying thefocal length of the second optical component, the control means beingcapable of controlling the first and second variable focal lengthreflective optical components so as to achieve a change in relativefocal length between the first and second components such that, in use,control over at least two optical parameters of the system is achieved.

The optical system preferably operates over a wide spectral range, forexample from 450 nm to 10,000 nm. A silvered mirror has a reflectivityof about 0.9 at a wavelength of 450 nm and a reflectivity of 0.99 at awavelength of 10,000 nm. The optical system has a focal plane which ispreferably at the same place for all wavelengths over which the systemis designed to operate. This is to be compared with optical systemsusing refractive optical components, where it is difficult to find amaterial for the components which has good transparency over a widespectral range, and such systems are generally achromatic i.e. theirfocal plane is the same for just a very narrow band of wavelengths.

The optical system is preferably light. Reflective optics offersignificant weight saving over equivalent refractive, perhaps 20%, or50% saving.

Each individual reflective optical component preferably has a high atransmission as possible, for example 0.95 or more, or 0.975 or more, or0.985 or more, even 0.99 or more. A compounded system compounds up thelosses and so if several optical elements are used it is desirable touse as many with high transmission as possible.

The optical system may further comprise an image plane fixed relative tothe first and second variable focal length reflective opticalcomponents.

The optical components, which are preferably reflective, may also beused to stabilise an optical beam incident on the system. This can beused to reduce the problem of handshake when the optical system is usedin a camera and/or reduce the effects of atmosphericaberration/compensate for other effects.

One or more of the reflective optical components may comprise a mirror.The control means may comprise a mechanical linkage, electronic circuitor a computer program. The or each reflective optical component may bedeformed to vary its focal length.

Variation of the focal lengths of the reflective optical components ispreferably carried out at high speeds, for example of the order of 10 Hzor more, 20 Hz or more, 40 Hz or more, or even 60 or 100 Hz or more.

One of the optical parameters controlled by the optical system may bethe magnification of the system. The magnification achieved by thesystem may be variable by a factor of 3 or more, or 5 or more, or even10 or more. The magnification may be variable between ×1 and, say, ×10or ×15, or above.

Tilt means, such as mirror manipulation means, may be used to apply atilt of at least one of the optical components about at least one axis.The tilt means may apply a two-axis-tilt to one or more of thereflective optical components, for example when magnification of theoptical system is controlled. This may ensure that the optical axis ofthe system is maintained during magnification.

The optical system may be used in a magnification, or zoom, unit. Themagnification unit may be used in conjunction with a camera.

Two or more optical systems may be used in conjunction with each other.One input may be supplied to such a ‘stacked’ optical system, the sameinput may be used to control both of the control means of the stackedsystem (or different control signals may be provided).

According to a sixth aspect of the present invention, there is providedan imaging system comprising an image detector and an optical systemaccording to any preceding aspect of the present invention.

The imaging system may further comprise de-focusing means to de-focusone or more of the, preferably reflective, optical components. Thisde-focuses the image on the detector, which minimises reflection fromthe detector surface. When the imaging system is to be used with alaser, this may also reduce the laser energy density at the detectorsurface and hence reduces the risk of damage of the detector by thelaser. The de-focusing means may be used to de-focus one or more of the,preferably reflective, optical components when the detector is not inuse. Control means may ensure that for a substantial part (e.g. at least¼ or ⅓ of the time or at least ½ of the time, or at least ¾ of the time)of the duty cycle of the detector (or detector array) the image incidentupon the detector is significantly de-focused (e.g. a nominal focusedspot may be de-focused to have an increase in area of 50% or more, 100%or more, 200% or more, 400% or more, 1,000% or more.

Means may be provided to detect an incident beam that is more intensethan a predetermined threshold and automatically de-focus the opticalsystem, at least for the pixel or pixels that would otherwise receiveradiation above the threshold intensity. The entire image may bede-focused in response to a signal of too great an intensity.

It may be desirable to have none, or only one or two refractivecomponents in the beam path (they are less transmissive than reflectivecomponents).

In order to minimise the risk of damage to a detector pixel in adetector array an incident image may be de-focused practically all ofthe time, without significant loss of image quality from the detectorarray if the degree of de-focus is controlled to match the systemresolution. The output resolution of a pixellated array is controlled inpart by the pixel size and geometry—there is little benefit in havinginput optical resolution better than the pixel output resolution. It ispossible to take advantage of this by having the optics deliberatelyde-focus the image incident onto the detector array to the extentconsistent with not degrading too much the output signals/picture of thearray. FIG. 10 illustrates this. For any particular level ofmagnification the degree of desirable de-focus may be different—athigher magnification less deliberate de-focus may be desirable asresolution may already be challenged.

According to another aspect the invention comprises an infra-reddetecting device comprising an infra-red detector and an optical systemwhich comprises at least a first and a second variable focal lengthoptical components, preferably reflective components, whose positionsare fixed relative to one another, and control means for varying thefocal length of the first or second variable focal length opticalcomponent in relation to the focal length of the other of the saidvariable focal length optical components, such that, in use, controlover at least two optical parameters of the optical system is achieved.

Preferably the device is an I.R imaging device, and it is preferably athermal I.R. device. Thermal wavelengths may be considered to be about 3to 14 microns.

The optical parameters controlled are preferably the focal lengths ofthe first and second optical components.

The thermal imaging system may further comprise reference means forproviding a reference signal to the detector. De-focus means may beprovided for controlling one or more of the optical components so as tocause the image of the scene received by the detector to be de-focused.

The de-focusing means may comprise the referencing means. If the sceneimage is substantially completely de-focused, the de-focused image ofthe scene may be indicative of the mean background temperature of thescene. It has long been desirable to produce a reference signal(periodically) that has a relationship with the captured scene, such asa signal indicative of the mean scene temperature, but hitherto thereference signal from mechanical choppers have typically been indicativeof the camera temperature. Each pixel of a detector array of pixels willreceive a mean scene temperature input if the scene image is fullyde-focused (e.g. the input of the system is focused onto the detectorarray).

The infra-red imaging system may alternatively or additionally comprisedither means to manipulate the first or second optical component (orfirst and second optical component) between two configuration(s) so asto cause the detected image to dither between two positions on thedetector. The dither means may change the shape of the opticalcomponent, e.g. mirrors, at a rate of 10 to 50 Hz, preferably 20 Hz±10Hz, or 30 Hz±20 Hz. This allows two slightly differently focused imagesof the scene to be incident on the detector. This is conventionallyachieved by using the two prism sections of a chopper device. This isA.C. coupling of the scene signal. It is important in some I.R. imagingsystems to A.C. couple the detector signals to reduce noise.

It is also possible to control the shape of the two optical componentsso as to compensate for image aberration/camera shake/other effectswithout the need to have additional specific dedicated compensatingoptics—i.e. the same optics that controls the magnification/zoom can beused to compensate for other effects. This can reduce weight andcomplexity.

There may be only two optical elements of variable focal length on theapparatus. There may be no other focusing optical elements.Alternatively, one or two (or more) additional optical components(reflective or refractive) may be provided for example to compensate foraberration and/or wide angle light collection (e.g. a Schmidtcorrector).

According to another aspect, the invention comprises a method ofproviding A.C. coupling (or dither) in an electronic imaging detectorcomprising controlling a mirror (or lens) upon which radiation from theincident scene is incident so as to at one point in time direct an imageto one position on an imaging detector, moving the mirror so as todirect the image at a second point in time to a different position onthe imaging detector, and moving the mirror so as to repeatedly ditherthe image on the imaging detector, and in which the mirror (or lens) isdistorted to achieve the effect rather than being bodily movedlongitudinally or bodily rotated.

Preferably the mirror (or lens) is controlled so as to direct de-focusedradiation from the scene onto the imaging detector. Preferably thede-focused radiation is substantially completely de-focused so as topresent mean scene radiation to the detector. Preferably the mean sceneradiation is periodically directed on the detector and is used as areference in processing detected signals from the detector array.

The mirror may also be used to focus the image onto the imagingdetector. The mirror may be part of zoom magnification optics and may beused at times to zoom the magnification of the image incident upon theimaging detector.

The method may also preferably comprise providing another focusingoptical element in addition to the mirror (or lens) having the mirror,other focusing element and imaging detector fixed distances from eachother, zoom being achieved by varying the focal length of the otheroptical element and the mirror (or lens).

The mirror may also be used to compensate for vibrations or movement.The other optical element is preferably bigger than the mirror and maybe moved or perturbed less often, or at a slower rate, than is thesmaller mirror. Radiation may be incident upon the other optical elementbefore it is incident upon the mirror.

According to another aspect the invention comprises a method ofproviding a periodic reference signal in an electronic imaging detectorcomprising operating the detector in a mean scene mode from time to timein which a mirror (or lens) is controlled to de-focus completely, orsubstantially completely, radiation from a scene so that radiationincident upon the imaging detector is uniform, and using a signalproduced by the detector during this mean scene radiation mode ofoperation as a mean scene reference signal, and controlling the mirror(or lens) to return to an imaging mode of operation after the referencehas been obtained.

The detector preferably switches to mean scene reference mode with aregular periodicity, preferably of the order of many times per second.

According to another aspect, the invention comprises a method ofminimising damage to detectors in an optical system and retro-reflectionfrom an optical detector system, the method comprising operating opticalfocusing components of the system in a de-focus mode in which thecaptured radiation incident upon a detecting element is significantlydefocused.

BRIEF DESCRIPTION OF THE DRAWINGS

Several embodiments of the invention will now be described, by way ofexample only, with reference to the drawings, in which:

FIG. 1 shows, schematically, a variable magnification optical systemaccording to the invention;

FIG. 2 shows, schematically, an alternative variable magnificationoptical system according to the invention in which the opticalcomponents are deformable mirrors;

FIG. 3a shows a graph of the required mirror deflections for the systemas shown in FIG. 2;

FIG. 3b shows an effective mirror boundary at the area where the imagespot ends (the position of the remaining outer portions of the mirrorbeing irrelevant so long as the portions where the image is incident arein the correct place);

FIG. 4 shows a beam expander according to the invention comprising twooptical components;

FIG. 5 shows a beam expander according to the invention comprising fouroptical components;

FIG. 6 shows schematically an infra-red imaging camera;

FIG. 7 shows schematically a prior art chopper for an I.R. camera;

FIG. 8 shows schematically the operation of an infra-red imaging system;

FIG. 9 shows schematically the difference between a de-focused image anda totally de-focused image; and

FIG. 10 shows schematically how a system operating at low magnificationcan be run in a de-focused mode without significant image degradation.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

FIG. 1 shows a compound lens system 1 according to the inventioncomprising a first lens 2 of variable focal length f₁ and a second lens3 of variable focal length f₂. The positions of the lenses 2,3 are fixedrelative to one another and with respect to the image plane 4 of thesystem. The spacing between the lenses, d, is therefore fixed, as is thedistance, S₂, from the plane of the second lens 3 to the image plane 4.As a result of fixing the positions of the lenses 2,3 and the imageplane 4 relative to one another, for any value of f₁ there is only onepossible value of f₂ that will provide an in focus image at the imageplane 4. For any particular value of f₁, the following equations can beused to calculate f_(eff), the effective focal length of the system, f₂and m, the angular magnification of the system. $\begin{matrix}{f_{eff} = \frac{f_{1}S_{2}}{f_{1} - d}} & \text{Equation~~1} \\{f_{2} = \frac{S_{2}( {f_{1} - d} )}{f_{1} - d - S_{2}}} & \text{Equation~~2} \\{m = \frac{f_{1}( {S_{2} - f_{2}} )}{f_{1}S_{2}}} & \text{Equation~~3}\end{matrix}$

These equations can be used to calculate the parameters of a system forany particular values of d and S₂.

For the purposes of illustration it is useful to consider the operationof a system in the region where the first lens 2 focuses the lightapproximately onto the image plane 4, without any help from the secondlens 3, i.e. where f₁=d+S₂. In this region the required range of focallengths f₂ of the second lens 3 is small.

To describe this region, it is easiest to first consider the situationwhere f₁=d+S₂ and then consider the consequences of either increasing,or decreasing f₁ by a small amount. For f₁=d+S₂, the intermediate focusof the system is on the image plane 4 and no modification of the lightpath is required by the second lens 3. The focal length f2 of the secondlens 3 is, therefore, equal to or substantially equal to infinity. Asmall increase in f₁ would place the intermediate focus just beyond theimage plane 4, therefore, requiring f₂ to go slightly positive tore-focus the image onto the image plane 4. Conversely, if f₁ isdecreased by a small amount, f₂ must go slightly negative to compensate.

Considering a system according to FIG. 1 having the values of d and S₂fixed at 150 mm and 20 mm respectively, it can be shown from equations 1to 3 above that for a range of f₁ between 160 mm and 180 mm, f₂ willvary from −20 mm through infinity to +40 mm and that the system willproduce a variation in magnification from −6 to −16, a change ofmagnification, at the fixed image plane, of nearly 3.

The system shown in FIG. 1 can be implemented using any suitablevariable focal length optical components. FIG. 2 shows a system 5utilising two reflective optical components 6,7 arranged in a Cassegraindesign. The system shown uses a large, deformable, positive primarymirror 6 to focus the light onto a smaller secondary mirror 7, placed infront of the primary mirror 6 to partially obscure the view. Thesecondary mirror 7 is substantially flat with the capability to deformeither to create a positive, or negative focal length mirror. A detector8 is positioned at the image plane in front of the primary mirror 6, inline with the secondary mirror 7. The system also comprises controlmeans 13 which controls the focal length f₂ of the secondary mirror 7 inrelation to the focal length f₁ of the primary mirror 6 in order tomaintain the correct interrelationship between f₁ and f₂ so as tomaintain focus on detector 8 for any given value of f₁. The controlmeans may comprise a mechanical linkage, an electronic circuit or acomputer program controlling the means used to deform the mirrors 6,7.

For a given system the required deflections of the mirror edges toproduce a desired performance can be calculated from a knowledge of thediameters of the mirrors to be used and their required radii ofcurvature, using the following equation: $\begin{matrix}{z = {R - {R\quad {\cos ( \frac{c}{2R} )}}}} & \text{Equation~~4}\end{matrix}$

where z is the required deflection, R is the radius of curvature of themirror and c is the diameter of the mirror when flat, i.e. when R=∞. Theradius of curvature of a spherical mirror is related to its focal lengthas follows:

 R=2f  Equation 5

FIG. 3 is a graph of the required effective boundary mirror deflectionsin a system according to FIG. 2 again considering the range f₁ between160 mm and 180 mm and with values of d and S₂ fixed at 150 mm and 20 mm.The effective diameters of the mirrors are 100 mm for the primary mirror6 and 20 mm for the secondary mirror 7. It will be appreciated that adeformable mirror typically has additional mirror surface beyond theeffective boundary, and that so long as that part of the mirror that theimage reflects from is in the correct configuration any additionalperipheral mirror surface is irrelevant, and can be at any position.

If the figures from the graph in FIG. 3 are included with parameterscalculated previously for f₂ and m in relation to the system shown inFIG. 1, the following numerical description of the behaviour of thesystem at the extremes is obtained.

Deflection Deflection required of required of Magni- mirror 6 on mirror7 on Focal fication boundary of boundary of length of Focal length of ofcompound optical beam optical beam mirror 6 mm mirror 7 mm system (inmm) (in mm) 160 −20 −16 3.90 −1.24 180 40 −6 3.47 0.42

The system described above provides for a compact, versatile imagingsystem whilst utilising mirrors of dimensions which can provide andwithstand the required deflections.

Although the above embodiments have been described by reference to theregion where f_(1≈)d+S₂, the invention is not limited to application inthis region but encompasses any application falling within the terms ofthe claims. For example in certain applications it may be desirable tocompletely de-focus the system. This could be achieved. for example, byadjusting the system to meet the condition that f₁+f₂=d.

FIG. 4 shows a beam expander according to the invention. The beamexpander comprises a negative primary mirror 9 and a positive secondarymirror 10 arranged in a Cassegrain system.

In the beam expander shown in FIG. 4, if the distance, d, between themirrors is fixed then it can be shown that the focal length f₂ of thesecondary mirror 10 is related to d and the focal length f₁ of theprimary mirror 9 by the relationship:

f ₂ =d−f ₁  Equation 6

and that the magnification, m, of the system is given by:$\begin{matrix}{m = \frac{f_{2}}{- f_{1}}} & \text{Equation~~7}\end{matrix}$

In the system shown in FIG. 4 a negative primary mirror 9 was chosen inorder to provide for a positive magnification with no image inversion inaccordance with equation 7. The range of magnification possible in asystem according to this design is limited by the ability to producemirrors of a suitable size which can provide and withstand the requireddeformations. In order to provide for wider ranges of magnification itis possible to use more than two optical components. For example, FIG. 5shows an expander comprising four optical components 9,10,11,12 whichacts like two stacked two optical element expanders. If the distancesbetween the primary and secondary mirrors are set the same for eachexpander and the radius of curvature for each of the primary mirrors9,11 and each of the secondary mirrors 10,12 is set the same then eachexpander will provide the same magnification and the primary mirrors9,11 and the secondary mirrors 10,12 may then be respectively variedidentically. In this type of arrangement the primary mirror 11 of thesecond expander may be same size as the secondary mirror 10 of the firstexpander.

It can be useful to have stacked optical elements (expander or focusingoptics) with the stacks of substantially the same radius of curvatureand spacing so that both sets of primary and secondary optics (or allsets if there are more than two sets) can be controlled using the samecontrol signals, so there is no need to calculate and generate aseparate set of control signals for the optical elements in each stack.This is, of course, achieved by selecting the geometry of the stacksappropriately and the size and focal length of the optical componentsappropriately.

In order to produce a similar range of magnification using a fouroptical component expander rather that a two optical component expander,the distances between the primary and secondary mirrors for eachexpander can be half that of the required distance for the two opticalcomponent expander, thus the overall length of a four optical componentexpander may be similar to that of an equivalent two optical componentexpander.

The concept of using stacked expanders can be extended to any number ofstacked expanders giving wider ranges of magnification.

FIG. 7 illustrates schematically a conventional infra-red thermalimaging system in which the infra-red detector elements 80 need toreceive a reference signal periodically, or be relieved of the presenceof the image (referenced 82) periodically. A ferroelectric array isnormally A.C.-coupled and has a chopper to do this. A bolometric arrayis normally D.C.-coupled, but still has a chopper to provide a periodicreference signal, typically for calibration.

In either a ferroelectric or bolometric array system, a rotatingmechanical chopper device 84 is typically placed in the thermal beam 86incident from the scene being imaged. The chopper typically comprisestwo transparent prism sections 88 and 90 (focusing an image to offsetpositions) and third opaque sections 92. The opaque sections present ablack body reference temperature signal to the detector. As the chopperis rotated, the detector periodically receives the reference temperatureof the opaque section and this effectively resets the detector. Adisadvantage of this system is that the temperature of the opaquesection is indicative of the temperature of the imaging system, and notof the scene being imaged. It can be seen that the imaging system of thepresent invention overcomes such a disadvantage, the referencetemperature signal used therein being a de-focused image of the sceneand, therefore possibly being indicative of or related to the averagetemperature of the scene.

The optical system may have one of the first or second opticalcomponents provided as a substantially flat mirror (or lens) which canbe manipulated to have a positive or negative curvature, and which canpreferably move from positive to negative curvature and back again,under the control of control means. Said one mirror (or lens) may besignificantly smaller than the other of said first and second opticalcomponents. It may be 50% of the diameter or less, or 30% or less, or20% or less, or 15% or less.

FIG. 6 shows schematically an infra-red thermal imaging systemcomprising a camera 60 comprising a detector array 62 of pixels 64; anoptical unit 65 comprising a primary mirror 66 and a secondary mirror68; and controller 70. A lens unit housing is provided, but not shown.Signals from the detector array 62 are fed to a signal processing unit72. Mirror perturbation/distortion means 74 and 76 are provided tocontrol the shape and configuration of the mirrors 66 and 68, under thecontrol of the control means 70.

The primary mirror is controlled, in combination with the secondarymirror 68, to achieve a desired magnification of the image as describedearlier. This may typically be ×10, or ×15, or so, or anywhere in therage ×1 to ×10 or ×20.

The system is chopperless. The function of the chopper is performed bythe variable mirrors 66 and 68. In this example mirror 68, the smallersecondary mirror, is alone moved by the distortion means 78 to achieve adither of the image on the detector (A.C.-couple the detector signals).This is typically achieved by moving the image bodily on the detector,for example whilst still in focus by, for example, a few pixels. It iseasier to move the smaller mirror, and movements of the smaller opticalelement can be made faster. Alternatively the image may be de-focused,and dithered in de-focused mode.

Mirror 68 is also distorted/moved, possibly alone or in combination withmirror 66, so as to de-focus the image. The image may be periodicallyde-focused, de-focused from time to time, or may be generally de-focusedall of the time, possibly with exceptions. This removes the focusedincident light from the pixels 64. In a conventional choppered system,the pixels would receive substantially black body radiation from thechopper/camera. In this embodiment the pixels 71 receive radiation fromthe scene, but deliberately de-focused. When the scene is substantiallytotally de-focused the pixels receive signals indicative of the meanscene temperature.

FIG. 9 illustrates the fact that the system may substantially totallyde-focus the scene image so that substantially no spatial information ispresent in the beam incident on the detector array—each pixel receivessubstantially the same intensity light. Although the paper by Butler etal referred to earlier says that this might be achieved by having athick transparent section in the mechanical chopper, it is difficult tosee how, for a pixel array of thousands of pixels, say, 256×256 pixels,or 512×512 pixels, any practical chopper can be made thick enough tototally blur the image to the extent necessary to have every pixelreceiving substantially the same intensity light. The chopper would beenormously thick.

FIG. 9 also illustrates a de-focused image regime, where it is stillpossible to tell that there is an image, and where on the sensor arrayit is, approximately, but it is not a sharp image, and is of lowerintensity than if it were focused. It may be possible to track ade-focused image (but not a totally de-focused image) and it may bepossible to evaluate where the centre of the de-focused image is.

Of course, it is possible to control the mirrors to direct the incidentlight away from the detector, getting closer to directly reproducing theeffect of a chopper.

The mirrors 68 and/or 66 may be controlled so as to present a referencesignal or image to the detector from time to time, possibly atpredetermined regular intervals. This reference may be the substantiallytotally de-focused scene image, and may give a mean scene temperaturereference.

It will be appreciated that it may only be necessary to provide onemirror (or in other embodiments optical element such as a lens) toprovide dither and/or de-focus, especially if zoom is not required. Zoommay be achieved by a more conventional mechanism and dither and/orde-focus by a controlled mirror (or optical element).

It will also be appreciated that in an infra-red imaging system we mayuse variable controllable transmissive lenses instead of mirrors, or amixture of variable transmissive and reflective variable opticalelements.

It will be appreciated that the deformable mirror 7 in FIG. 2 can beused in an I.R. imaging system to provide an accurate and fast trackingmechanism which can keep an area of the obscured scene of interestcentred in the image field during a zoom procedure, and can also be usedfor tracking a moving object (and also to compensate for jitter, ormovement of a platform upon which the system/camera is mounted).

As discussed elsewhere, the two mirrors (optical elements) can be driventogether to produce a large, but controllable, deformation that can beused to achieve A.C. coupling of the detectors (e.g. thermal infra-reddetectors), and/or for mean scene temperature referencing in thermalimaging.

In the embodiments described above only the basic design requirementshave been outlined. These designs can be varied to include modifiedcomponents to correct for aberrations as is known in the art. Suchmodified components may, for example, include parabolic or ellipticalmirrors, or refractive or diffractive elements.

Variable focal length optical components for use in a system accordingto the invention can be produced for example by using deformablemirrors, refractive optical components, liquid crystals and non-linearoptical effects, but any variable focal length optical components can beutilised in a system according to the invention. If refractive opticalcomponents are used these could be of the inflatable type which can beat least partially filled with a high refractive index fluid, theoptical power of the element being varied by increasing or decreasingthe amount of said fluid in the element. Alternatively an elastic lenssimilar to the human eye could be used, wherein the lens curvature isincreased by applying a radial compression.

The invention may be used in infra-red imaging system, on I.R. sensors,or optical cameras (possibly CCD cameras, and/or video cameras)telescopes, binoculars, and other areas.

The use of all-reflective optics, with substantially no (or no)refractive optics provides a system with greater hyperspectralapplications. The system will transmit over a wide spectral bandpass,and have a common focal plane at all operational wavelengths. Thiscompares with the refractive systems, for which it is difficult to finda material which has good transparency over the whole spectral range,including visible, near, mid, and thermal infra-red.

In refractive systems it is necessary to have doublets or triplets thatare generally designed to be achromatic in the sense that their focalplane is the same at just two or three design wavelengths, and thendiffers as little as possible for other wavelengths in the chosenworking region. This effect can be exacerbated when using zoom/highmagnification facilities. For example, even television cameras whenusing extensive zoom can have a change in the colour of the image.

The above advantage can be used in infra-red imaging detectors,collimators, beam expanders, and indeed in visible optic systems such astelescopes, cameras and binoculars.

The use of the deformable surfaces (optical elements) is well suited toa two-axis tilt system which is provided on one or more of thedeformable surfaces. The purpose of a two-axis tilt system is typicallyto ensure that the optical axis of a zoom lens is maintained during thezoom function (compensation of image wander). Such control is difficultto implement in a refractive system without adding further opticalcomponents and hence introducing further image aberrations andincreasing the weight/complexity. It is an elegant solution to be ableto use the same reflective variable focus components that perform thezoom to perform the compensation. This applies to any wavelength system,including visible and infra-red. Using the same first and second opticalelements as are used for zoom to compensate for other things, such ascamera shake, centring the image during zoom, aberration, beam steering,etc. is an elegant solution. Similarly, it is elegant to use the samevariable focal length components to provide dither and a referencesignal in an infra-red camera, or an infra-red detector.

A particular interesting application is that in thermal imaging systemsit may be possible to distort the mirror to use one or two axis tilts asa mechanism for switching into the detector field of view one or morethermal reference sources that may be used for system calibration.

It will be appreciated that it is possible to control one (or more) ofthe reflective optical elements (mirrors) in an infra-red imaging systemto switch into a reference source mode, as well as provide a ditherand/or provide a de-focused image to the detector. The de-focused imagemay be the reference.

In all applications where microscan is to be used to increase the imageresolution by post-processing, or to be used with an A.C. couplerdetector, the implementation of fine-control, two axis pointing can beused, in combination with a zoom function if required. Again, the sameoptical components (refractive or reflective) can be used to perform themicroscan, zoom and to perform the two axis pointing along the beambeing detected/imaged.

As has been previously discussed, within the design of the reflectivezoom system it is relatively easy to change the curvature of onedeformable surface so as to produce a substantial and rapid shift inimage focus without the need to move any optical component bodily.Because of the speed of operation and the substantial nature of thefocus shift that may be implemented, such control may be exploited in anumber of ways.

Firstly, in all imaging systems (of any wavelength) it may be desirableto minimise the possibility of a retro-reflection from the detectorsurface, and the image may be de-focused by a controllable amount duringroutine operation. Thus, a detector does not normally receive a focusedimage but rather a slightly de-focused image. This reduces theretro-reflective strength by spreading the reflective radiation over anincreased and controllable angular range. The use of spectral,polarisation, or other forms of discriminant in a de-focused image maybe used to detect the presence of an object sought in the image. Onlywhen the object sought in the image is to be confirmed as indeed anobject of the desired class, need the system be focused and zoomed toprovide the high-quality image required for image recognition andidentification of the object to be sought. This is schematicallyillustrated in FIG. 8. After locating a positive match for theobject/identifying it, the imaging system may be coupled to otheroperational systems. It may be possible to track a tagged object whilstin de-focused mode.

The system may always keep the image de-focused to some degree.

Thus, an imaging system may have a de-focused mode of operation duringroutine operation, with reduced retro-reflection from the detectorsurface, and then when the detected image signal, which is processed,identifies the presence of an object, possibly of a predetermined classof objects (and possibly not), the system may focus the image andpossibly also zoom in on the image/on the object to provide a muchhigher quality image. This higher quality image, and possibly enlargedimage, may then be used by the system for pattern recognition/objectidentification. For example, it may be compared with a series ofreference images in, for example, a pattern recognition correlator.

By having a de-focused image during normal operation, unexpected flashesof light may damage the individual detectors/pixels less than would bethe case with a focused image, and as mentioned above, theretro-reflection from the detector surface is minimised.

A second application is where the detector is vulnerable to damage by anunexpected flash, for example lightening or a laser. Again, the systemmay be de-focused to reduce the energy density at the detector surface.

Where the system is used at less than the maximum zoom and the detectorpixel size is larger than the optical resolution of the system, thede-focus may be chosen to match the pixel-defined resolution. The systemin which the detector pixel provides defraction-limited resolution at,for example 5×zoom could be operated at, for example 1×zoom, with ade-focus that increased the area of focus spot by a factor of 25, with acorresponding reduction in energy density in the focus beam. This can beused to protect the detector. The de-focus may be the square of the zoomfactor. It may be desirable to de-focus to reduce the intensity by afactor of the order of 50 or more, or of the order of 100 or more.

FIG. 10 shows how a small intense beam spot 100 can be de-focused tofill (or substantially fill) the pixel without significant loss ofresolution: the pixel still receives the same amount of light, and stillemits substantially the same signal with de-focus, but the intensity oflight on the pixel surface is reduced, and this may prevent the pixelsurface from becoming damaged by too intense a spot. The wider field ofview of the lower magnification is maintained.

In all applications where the detector duty cycle is less than 100%, thesystem may be fully de-focused at all times when the detector is notintegrating a signal (i.e. is not in effective use). This provides bothreduced retro-reflection and reduced vulnerability to unexpected highintensity flashes.

In many I.R. imaging sensors (and visible electronic cameras) thescene-coupling ability of the system is not the limiting factor to readout/display/detection. A scene image may be captured by a detector arrayin, say, 20 ms, but it can take three times that long to read out thevalues from the detector pixel array—the “eye” of the system is onlyneeded for a fraction of the overall time. However, if the “eye” remainsopen the pixel detectors are vulnerable to damage from too brightflashes. It is proposed to de-focus the image incident on the detectorswhen they are not integrating a signal (or possibly even divert theimage elsewhere).

The above applications are viable because of the speed and flexibilityafforded by the reflective system over and above refractive systems.

Refractive optical elements are generally heavier than the reflectiveequivalent, and thus the use of a reflective zoom provides a system ofreduced mass compared to its refractive counterpart.

A further advantage of reflective systems over refractive systems isthat in thermal imaging systems Germanium, Zinc Selenide and othertransparent materials used to manufacture transmissive elementstypically have a transparency of about 90%. Replacement of one or moreof those elements with a reflective element (for example having a Goldcoating) can produce a reflector with an efficiency of 99% (or aboutthat), and can thus lead to a useful increase in overall systemtransmission. An-all reflective optic system would have even moreadvantages.

It will be appreciated that it is possible to change the first opticalcomponent (e.g. mirror) and/or the second optical component (e.g.mirror) from having one of a positive or negative focusing power to theother of a positive or negative power, in use, possibly whilst changingthe magnification of the image.

The control means may be a microprocessor or computer. It may beprogrammable. The user may be able to change the program that isoperating at any particular time—i.e. user-selectable programme.

Although this specification discusses pixellated detector arrays, andthese would typically have hundreds and thousands of pixels, it isenvisaged that non-pixellated detectors would benefit from the inventionand it is intended to protect those. Furthermore, a detector, as opposedto an imager, may have just a single detection “pixel”, or no imagingfunction, and again protection is sought for the application of thepresent invention in that area. The definitions of protection should beinterpreted accordingly.

What is claimed is:
 1. An infrared imaging system for providing avariable IR image, said system comprising: an infrared detector; and anoptical system for directing the image onto said infra-red detector,said optical system comprising: at least first and second variable focallength reflective optical components whose positions are fixed relativeto one another, and control means for varying the focal length of thefirst and second variable focal length optical components to direct theimage onto the detector in a controlled manner.
 2. An imaging systemaccording to claim 1 in which the control means is capable ofcontrolling the first and second optical components so as to provide afocused image on the detector whilst altering the magnification of theimage.
 3. An imaging system according to claim 2, which comprises meansfor relieving the detector of the focused scene image.
 4. An infraredimaging device according to claim 1, in which at least one of the firstand second optical components is controlled by the control means tode-focus the image incident upon the detector.
 5. An imaging systemaccording to claim 4 in which the de-focused image is substantiallycompletely de-focused so as to be indicative of a mean scene temperatureof the image.
 6. An imaging system according to claim 5 in which themean scene temperature is used as a reference.
 7. An imaging systemaccording to claim 1, which comprises dither means adapted to dither theimage focused onto the detector between different dither positions onthe detector.
 8. An imaging system according to claim 7 in which thedither means comprises one of the first and second optical componentscontrolled by the control means.
 9. An imaging system according to claim1 in which the control means ensures that for a substantial part of theduty cycle of the detector array the image incident upon the detector issignificantly de-focused.
 10. An imaging system according to claim 1 inwhich the first and second reflective optical components are bothadapted to have their focal lengths varied by one of deflection anddistortion.
 11. An imaging system according to claim 1 in which at leastone of the first and second optical components are controlled by thecontrol means so as to compensate for aberration and/or vibration. 12.An imaging system according to claim 1 in which the only focusingelements are the first and second variable focal length opticalcomponents.
 13. An imaging system according to claim 1 in which thefirst and second optical components are operatively stacked with atleast third and fourth variable focal length optical components so as toincrease the magnification that can be achieved by the system.
 14. Animaging system according to claim 13 in which the first, second, third,and fourth optical components are all reflective.
 15. An imaging systemaccording to claim 13 in which the third and fourth optical componentshave substantially the same focal lengths as the first and secondoptical components respectively, and are controlled by the control meansas are the first and second components.
 16. An imaging system accordingto claim 1 in which the first and second components and the detector arearranged in the Cassegrain arrangement.
 17. An imaging system accordingto claim 1 which has control means adapted to control the first andsecond optical elements so as to operate the system in a de-focused modeof operation at times and in which the control means is adapted todetermine when a possible object of a predetermined class is present inthe image and dependent upon establishing that it is adapted to operatethe system in a focused mode of operation so as to have an image focusedupon the detector, and adapted to identify the object using the detectedfocused image.
 18. An imaging system according to claim 17 which isadapted to return to the de-focused mode of operation once an object hasbeen identified.
 19. An imaging system according to claim 1 capable ofzooming the magnification of the image incident upon the detector and inwhich the control means is adapted to operate the system at a first,lower, magnification state and a second, higher, magnification state,and in which the control means operates the system at the first, lower,magnification state with the image de-focused to increase the area ofany nominal focused area, thereby preserving the larger field of view ofthe lower magnification state but ensuring that energy density from apoint source is equivalent to the higher magnification state.
 20. Animaging system according to claim 19 in which the de-focus is by afactor which takes the resolution of the optical components of thesystem to about the resolution of the overall output of the display, soas to avoid significant image degradation.
 21. An imaging systemaccording to claim 1 in which the image is arranged to be de-focused atsubstantially all times that the detector is not integrating a signal.22. An imaging system according to claim 1 at least one of said firstand second optical components comprise a substantially flat mirrorresponsive to manipulation by distortion means under the control of thecontrol means to focus incident light on a fixed image plane by movingthe mirror so as to produce either a positive or a negative focusingpower.
 23. An imaging system according to claim 22 in which the mirrorcan be moved from having a positive focusing power to having a negativefocusing power.
 24. An imaging system according to claim 22 in which onesubstantially flat mirror is significantly smaller than the other ofsaid first and second mirrors.
 25. An optical system comprising: atleast two variable focal length reflective optical components whosepositions are fixed relative to one another; and control means forvarying the focal length of said at least two variable focal lengthreflective optical components to control an image passing through saidsystem.
 26. An optical system according to claim 25 which operates overa spectral range, for example between 450 nm to 10,000 nm.
 27. Anoptical system according to claim 26 which has substantially the samefocal plane for all wavelengths over which it operates.
 28. An opticalsystem according to claim 25 which comprises an image plane fixedrelative to the variable focal length reflective optical components. 29.An optical system according to claim 25 in which the reflective opticalcomponents are also used to stabilise an optical beam incident on thesystem.
 30. An optical system according to claim 25 in which variationof the focal lengths of the reflective optical components is carried outat speeds of 10 Hz or above.
 31. An imaging system comprising an imagedetector and an optical system according to claim
 25. 32. An opticalsystem for varying the width of an optical beam incident on the system,comprising: at least two variable focal length optical components whosepositions are fixed relative to one another; and control means forvarying said at least two variable focal length optical components suchthat the width of said optical beam is varied whilst substantiallymaintaining any collimation of said optical beam.
 33. An optical systemaccording to claim 32 in which the optical components comprisereflective optical components.
 34. A method of dithering an imagebetween first and second positions on an imaging detector, the methodcomprising the steps of: controlling a variable focal length opticaldevice upon which radiation is incident so as to direct an image to oneposition on an imaging detector, distorting the device so as to directthe image to a different position on said imaging detector, andrepeating the controlling and distorting steps so as to repeatedlydither the image on the imaging detector.
 35. A method according toclaim 34 in which the optical device is controlled so as to directde-focused radiation from the scene to the imaging detector.
 36. Amethod according to claim 35 in which the de-focused radiation issubstantially completely de-focused so as to present mean sceneradiation to the detector.
 37. A method according to claim 36 in whichthe mean scene radiation is periodically directed on the detector and isused as a reference in processing detected signals from the detectorarray.
 38. A method according to claim 34 in which the optical device ispart of focusing optics and is also used at times to focus the imageonto the imaging detector.
 39. A method according to claim 38 in whichthe optical device is part of zoom magnification optics and is used attimes to zoom the magnification of the image incident upon the imagingdetector.
 40. A method according to claim 34 comprising providinganother focusing optical element in addition to the optical device andhaving the optical device, the optical element and imaging detector atfixed distances from each other, zoom being achieved by varying thefocal length of the optical element and the optical device.
 41. A methodaccording to claim 40 in which the optical element is bigger than theoptical device and is moved or perturbed less often, or at least aslower rate, than is the smaller optical device.
 42. A method accordingto claim 40 in which radiation is incident upon the optical elementbefore it is incident upon the optical device.
 43. A method according toclaim 34 in which the optical device is also used to compensate forvibrations or movement of an imaging device.
 44. A method of providing aperiodic reference signal in an electronic imaging detector, said methodcomprising the steps of: operating the detector in a mean sceneradiation mode from time to time by varying a focal length of a variablefocal length optical device to change its focal length so that radiationincident upon the imaging detector is uniform and has substantially nospatial information, using a signal produced by the detector during thismean scene radiation mode of operation as a mean scene reference signal,and controlling the focal length of the device to return to an imagingmode of operation after the reference has been obtained.
 45. A methodaccording to claim 44 in which the detector switches to mean scenereference mode with a regular periodicity, preferably of the order ofmany times per second.
 46. A method of minimising damage to detectors inan optical system and retro-reflection from an optical detector system,the method comprising the steps of: operating variable focus opticalcomponents of the system in a de-focus mode by varying the focal lengthof said optical component such that the captured radiation incident upona detecting element is significantly de-focused.
 47. An infrared imagingsystem for providing a variable IR image, said system comprising: aninfrared detector; and an optical system for directing the image ontosaid infra-red detector, said optical system comprising: at least firstand second variable focal length reflective optical components whosepositions are fixed relative to one another, and control means forvarying the focal length of at least one of said first and secondvariable focal length optical components in relation to the focal lengthof the other of the said variable focal length optical components suchthat, in use, control over at least two optical parameters of theoptical system is achieved, and in which said at least one of the firstand second optical components are adapted to be adjusted by one ofdistortion and deflection in order to vary their focal length, whereinthe first and second optical components are operatively stacked with atleast third and fourth variable focal length optical components so as toincrease the magnification that can be achieved by the system and thethird and fourth optical components have substantially the same focallengths as the first and second optical components, respectively, andare controlled by the control means as are the first and secondcomponents.